Always call update_use_power() instead of directly setting the use_power var.

- This will let us react to the change appropriately.
- While we're here, lets define some constants so we can stop using bare numbers.
- Always use those constants when calling update_use_power()

code/__defines/machinery.dm

@@ -11,6 +11,11 @@ var/global/defer_powernet_rebuild = 0      // True if net rebuild will be called
 #define DOOR_CRUSH_DAMAGE 20
 #define ALIEN_SELECT_AFK_BUFFER  1    // How many minutes that a person can be AFK before not being allowed to be an alien.
 
+// Constants for machine's use_power
+#define USE_POWER_OFF    0	// No continuous power use
+#define USE_POWER_IDLE   1	// Machine is using power at its idle power level
+#define USE_POWER_ACTIVE 2	// Machine is using power at its active power level
+
 // Channel numbers for power.
 #define EQUIP   1
 #define LIGHT   2